What is the history and (current) location of the root text sources comprising the Pali Cannon?

The oldest extant complete Pāli Canon in manuscript form is from 1500 AD, 2000 years after the Buddha, but that this collection preserves older text is well-known, because of the amount of parallels it has with older literature, such as that preserved in China.

https://books.google.ca/books?id=8jPYUCy-GxQC&pg=PA4&redir_esc=y#v=onepage&q&f=false

There used to be a very old Pāli Tipiṭaka housed at Aluvihare Rock Temple in Sri Lanka, but it was destroyed during the Rebellion of 1848.

More on the poor state of preservation of Pāli manuscripts (largely due to war, poverty, and climate) here: http://www.academia.edu/6508616/Pali_Manuscripts_of_Sri_Lanka

Also in the above article you will find recounted the tragic destruction of most ancient Sri Lankan manuscripts, which had to be reconstructed with Burmese and Thai manuscripts.

Compare this with the Sarvāstivāda Saṃyuktāgama, Lesser Sarvāstivāda Saṃyuktāgama, Dharmaguptaka Dīrghāgama, & related vinaya texts from the 팔만 대장경 (Palman Daejanggyeong, or the “Goryeo/Koreana Tripiṭaka”), which is from the 1200s. That is the oldest complete collection of EBTs currently extant. These texts were translated in China from an unknown Prākrit language (many speculate Gāndhārī) between 200 & 400 AD.

The Koreana Tripiṭaka is housed at Haeinsa Temple in Gayasan National Park, South Gyeongsang Province, South Korea.

Document collections older than that are highly eroded and fragmentary in nature, here is a link on the numerous śrāvakayāna & bodhisattvayāna Gāndhārī fragments that date from between 100 BC and 100 AD, with some from as late as 900 AD: http://www.historyofinformation.com/expanded.php?id=149

Please read the book "The Authenticity of the Early Buddhist Texts" (hard copy can purchased here) by Bhikkhu Sujato and Bhikkhu Brahmali.

The abstract states:

This work articulates and defends a single thesis: that the Early Buddhist Texts originated in the lifetime of the Buddha or a little later, because they were, in the main, spoken by the Buddha and his contemporary disciples. This is the most simple, natural, and reasonable explanation for the evidence.

Our argument covers two main areas:
1. The grounds for distinguishing the Early Buddhist Texts (EBTs) from later Buddhist literature;
2. The evidence that the EBTs stem from close to the Buddha’s lifetime, and that they were generally spoken by the historical Buddha.

Most academic scholars of Early Buddhism cautiously affirm that it is possible that the EBTs contain some authentic sayings of the Buddha. We contend that this drastically understates the evidence. A sympathetic assessment of relevant evidence shows that it is very likely that the bulk of the sayings in the EBTs that are attributed to the Buddha were actually spoken by him. It is very unlikely that most of these sayings are inauthentic.

Also, "A History of Mindfulness" (and here) by Bhikkhu Sujato (as suggested by Andrei Volkov).

Another good source is "History of the Buddhist Canon" by Daniel Veidlinger, who teaches Comparative Religion at the California State University.

Also: "On the Very Idea of the Pali Canon" by Steven Collins.

This chapter maybe interesting to you: "Recent Discoveries of Early Buddhist Manuscripts: And Their Implications for the History of Buddhist Texts and Canons" by Richard Salomon.

For a scholarly paper on early manuscripts, see "The Senior Manuscripts: Another Collection of Gandharan Buddhist Scrolls" by Richard Salomon. But this paper discusses only one set of manuscripts that was discovered.

Perhaps, it should be made clear that the oral tradition of the Pali Canon is much more important than the written version. Please see "Pali Oral Literature" by L.S. Cousins. I quote below:

Early Buddhist literature is an oral literature. Such a literature is not without its own characteristic features. A widespread use of mnemonic formulae is one of the most typical of these. I would refer to the considerable body of research on the nature of oral epic poetry. In such poetry the formulae are used both as an aid to actual performance and to maintain the continuity and form of the epic tradition.

Both these features are certainly present in the sutta literature. In the first place many suttas are clearly designed for chanting. We should assume that, then as now, their chanting would produce a great deal of religious emotion - the pamojja and piti-somanassa of the texts. The difference of course would be that the language of the suttas would still be directly comprehensible to the hearers. In these circumstances suttas would be chanted by individual monks both for edification and for enjoyment. We may compare the recitations attributed to Ananda and Upali in accounts of the First Council. In practice they would have to be tailored to the needs of the particular situation ~ shortened or lengthened as required. An experienced chanter would be able to string together many different traditional episodes and teachings so as to form a coherent, profound and moving composition.